Atherosclerosis is a chronic inflammatory reaction involving both the innate and adaptive immune system. The activation of the adaptive immune system requires the primary stimulus along with the ligation of costimulatory ligands and receptors. LIGHT/lymphotoxin is one set of costimulatory ligands. Signaling by these ligands is shown here to affect lipoprotein metabolism and atherosclerosis. Members of this family of ligands and receptors are expressed by immune cells that either have been shown to or are thought to influence the development of atherosclerosis as well as on non-immune cells such as hepatocytes. We have observed that manipulation of LIGHT/lymphotoxin and signaling through their receptors exert profound effects on plasma lipoproteins, yet these effects may be counterbalanced at the level of the vascular wall perhaps by effects on local vascular inflammation. Blocking LIGHT/lymphotoxin signaling in LDL receptor deficient mice using a soluble lymphotoxin beta receptor significantly decreased VLDL levels, but had no effect on the size of the aortic root lesion. LIGHT over expression in T-cells in LDL receptor deficient mice resulted in the accumulation of a small LDL, and a reduction in the size of the aortic root lesion. We propose three specific aims dealing with VLDL (aim 1), LDL (aim 2) and atherosclerosis (aim 3) following manipulation of LIGHT/lymphotoxin signaling. Specific aim 1 will examine the mechanism by which interfering with LIGHT/lymphotoxin influences VLDL metabolism, and whether this effect is mediated by LIGHT or lymphotoxin alpha1beta2, and the potential role of apoE. Specific aim 2 will examine the mechanism by which over expression of LIGHT influences LDL heterogeneity, including if insulin resistance has a role. The final aim will examine the role of this family of ligands on atherosclerosis, including if the LIGHT/lymphotoxin family influences plaque stability. The goal of this proposal is to understand how the LIGHT/lymphotoxin ligand family influences lipoprotein homeostasis and atherosclerosis. These studies will further our understanding of how the immune system influences these processes and may point to potential sites for therapeutic intervention.